1. Field of the Invention
This invention relates to the patterning of regions of material over chosen portions of semiconductor structures. In particular, this invention relates to plasma etching techniques for fabricating conductive layers of aluminum or chemically similar materials over selected portions of semiconductor structures.
2. Prior Art
Continuing improvements in semiconductor fabrication technology have caused the dimensions of some integrated circuit structures to be limited by the dimensions of aluminum or other metal lines formed on the surface of the structure to interconnect various portions of the underlying structure. Because the chemical reactions involved are not directional isotropic wet chemical etchants are not suitable for etching through thick metal lines while simultaneously accurately retaining lateral dimensions. One technique which has been developed to allow more accurate control of dimensions is etching of semiconductor structures with plasma. Various processes for plasma etching are now known and have been the subject for several patents. See, e.g., U.S. Pat. No. 3,615,956 entitled "Gas Plasma Vapor Etching Process"; U.S. Pat. No. 3,994,793 entitled "Reactive Ion Etching of Aluminum"; and U.S. Pat. No. 4,057,460 entitled "Plasma Etching Process."
Plasma etching processes have been developed which use chlorine based plasmas to remove aluminum with little or no undercutting of overlying protective materials, for example, photoresist. Unfortunately, after removing the aluminum such plasma processes also will etch any silicon beneath the aluminum. Because in commercial processing of large quantities of integrated circuits some or all of the integrated circuits will remain in the plasma etching reactor for some time after the aluminum has been removed from one of the circuits, and because the rate of removal of material by plasma etching is not necessarily uniform across even the surface of a single circuit being etched, a significant amount of etching of materials underlying the aluminum has been observed. For example, in some areas of the integrated circuit the aluminum will be completely removed from undesired regions in a very short time, while in other areas a much longer time will be required to remove the aluminum. Consequently in those areas where the aluminum is removed relatively quickly the plasma etching will remove underlying materials. This problem usually is most severe in areas of integrated circuits where metal connections make direct contact to underlying silicon, for example, the emitters of bipolar transistors or the sources or drains of MOS transistors. In these areas of the integrated circuit there are frequently PN junctions close to the surface of the silicon, and even shallow plasma etching of the silicon can expose or destroy the PN junction and ruin the function of the device.
Additionally, because silicon is soluble in aluminum, aluminum formed over silicon may dissolve portions of the silicon and seep into the underlying structure, creating spikes which degrade or destroy the operation of the integrated circuit. This problem has been partially overcome by the use of aluminum-silicon metal alloys which are saturated or nearly saturated solutions, in which solutions the underlying pure silicon will be much less soluble. Unfortunately, when the aluminum-silicon is chemically removed from the surface of the structure, some of the silicon from the aluminum-silicon mixture will remain on the surface of the structure. This residual silicon increases the roughness of the surface of the semiconductor structure, and makes difficult the reliable subsequent formation of metal conducting lines or other regions on the surface.
Therefore, one object of this invention is to provide a means of overcoming the difficulty of utilizing chlorine based plasma etching for patterning aluminum or chemically similar material layers on semiconductor structures. A further object of this invention is to eliminate the need for adding silicon to the aluminum or other material chosen to form the metallization layers.